Hydraulic Conductivity of a Firn Aquifer in Southeast Greenland

Abstract

Some regions of the Greenland ice sheet, where snow accumulation and melt rates are high, currently retain substantial volumes of liquid water within the firn pore space throughout the year. These firn aquifers, found between ∼10 and 30 m below the snow surface, may significantly affect sea level rise by storing or draining surface meltwater. The hydraulic gradient and the hydraulic conductivity control flow of meltwater through the firn. Here we describe the hydraulic conductivity of the firn aquifer estimated from slug tests and aquifer tests at six sites located upstream of Helheim Glacier in southeastern Greenland. We conducted slug tests using a novel instrument, a piezometer with a heated tip that melts itself into the ice sheet. Hydraulic conductivity ranges between 2.5 × 10 −5 and 1.1 × 10 −3 m/s. The geometric mean of hydraulic conductivity of the aquifer is 2.7 × 10 −4 m/s with a geometric standard deviation of 1.4 from both depth specific slug tests (analyzed using the Hvorslev method) and aquifer tests during the recovery period. Hydraulic conductivity is relatively consistent between boreholes and only decreases slightly with depth. The hydraulic conductivity of the firn aquifer is crucial for determining flow rates and patterns within the aquifer, which inform hydrologic models of the aquifer, its relation to the broader glacial hydrologic system, and its effect on sea level rise.

Near‐surface hydraulic conductivity of northern hemisphere glaciers

“…This also compares well, albeit an order of magnitude lower, to the recent 10 0 -10 2 m day −1 estimates for the hydraulic conductivity of firn on alpine glaciers (e.g., Fountain, 1989;Schneider, 1999) and the Greenland Ice Sheet (e.g., Miller et al, 2017). Our K-values are the same order of magnitude as those reported for ablating glacier ice by Cook, Hodson, and Irvine-Fynn (2016), and similar to the lower order estimates given by previous site-specific studies (e.g., Karlstrom et al, 2014;Larson, 1977;Theakstone & Knudsen, 1981;Wakahama, 1978;Wakahama et al, 1973).…”

Retrieval of Englacial Firn Aquifer Thickness From Ice‐Penetrating Radar Sounding in Southeastern Greenland

“…Similar to the previous study, we interpret the 0.2‐km upstream migration to be associated with lateral movement of englacial water driven by hydraulic head gradient of the firn aquifer. Using a hydraulic conductivity of 10 −4 (Miller et al, ), we estimate an aquifer migration up to 0.2 km in 2 years based on a specific discharge of 2 × 10 −6 m/s estimated using Darcy's law and the depth of water table provided by Forster et al ().…”

Direct Evidence of Meltwater Flow Within a Firn Aquifer in Southeast Greenland

“…Darcy's law predicts a specific discharge of 2.7 × 10 −6 m/s using a hydraulic conductivity estimated as 2.7 × 10 −4 m/s (Miller et al, ) and a hydraulic gradient, determined from ground penetrating radar (Forster et al, ), of 0.01 m/m (slope = 0.8°). The specific discharge measurements from the borehole dilution tests (mean specific discharge of 4.3 × 10 −6 m/s, standard deviation 2.5 × 10 −6 m/s) generally agree with the specific discharge predicted by Darcy's law.…”

“…Field observations of highly permeable firn (~10 −12 to 10 −10 m 2 ) and a sloping (less than ~2°) water table (Forster et al, ; Koenig et al, ; Miller et al, ) suggest that meltwater should flow within the aquifer if a connected network of pores is maintained. According to Darcy's law, the hydraulic conductivity and the hydraulic gradient within an aquifer control the specific discharge.…”